The idea is that you'd need to make the computational challenge more expensive to solve than any profit that success would have. This is what some DDoS protection services do (e.g. the "checking your browser" messages you might see).
In some scenarios such as spam, the profit per bot action isn't high at all, so this might be feasible. If it's a ticket bot, that wouldn't work at all.
Of course, you're hurting users without good devices, which just sucks.
I haven't really thought too deep into the economics of this, I don't know if it'd work at all. Just a thought.
Edit: Meant to include mobile in there, I don't know how if this would or wouldn't work with this scheme for mid-end+ Android devices or iPhones.
HashCash had the same idea and failed for the same reason: bad actors aren't using their own machines. They have access to the cheapest compute in the world: botnets and the compute of honest actors.
Add in the mobile-device issue and you quite literally have a solution that's lose/lose.
The thought is that a single user isn’t inconvenienced by a small amount of computational work to access a site, but a scraper is tarpitted into uselessness by it. Unfortunately this completely ignores the fact that the majority of site accesses are on battery power now.
